Planet X (place)

The original search for Pluto was motivated by unexplained orbital motions of Neptune and Uranus, which was assumed to be caused by "Planet X". Neptune had been found by examining the gravitational effect it had on Uranus. Because he was looking for Planet X, and through serendipity, Clyde Tombaugh discovered Pluto in 1930. However, Pluto is not large enough to explain the observed orbital motions. Tombaugh was just lucky. It has been 70 years, and while unlikely, the existence of Planet X is still a possibility.